In a mobile communication system, a base station receives signals from a plurality of user terminals. Two particularly attractive reception techniques for demodulating the symbols from multiple user terminals in each cell are successive interference cancellation (SIC) and interference rejection combining (IRC). Both of these reception techniques require the baseband receiver to estimate the channel between each user terminal and each antenna. The quality of the channel estimates greatly influences the performance of both SIC and IRC.
The network may receive signals from a user terminal at multiple network antennas. To receive a signal from a given user terminal, the network determines the set of network antennas that will be used to receive the signal transmitted from the user terminal. The signals received by this set of antennas are sent to a “uplink receiver” that demodulates the signal transmitted by the user terminal. Note that the same set of network antennas could be used for the reception of multiple user terminals.
The uplink receiver typically estimates the uplink channels between each user terminal and network antenna using reference signals that are transmitted from each user terminal on the uplink. In Release 8 of the LTE standard, at least one OFDM symbol in each 0.5 msec slot is devoted to the transmission of reference signals by all user terminals. The reference signal from each user terminal is transmitted over M consecutive sub-carriers. Assuming that M is greater than or equal to 72, the reference signal is a Zeda-Chu (ZC) sequence. There are 60 distinct base ZC sequences available in LTE.
When estimating the uplink channel from a particular user terminal, the reference signals from other user terminals act as interference and degrade the accuracy of the channel estimation. Therefore, it is generally desirable that the reference signals from all the user terminals be mutually orthogonal. In an LTE system, given one reference signal spanning consecutive subcarriers, a second orthogonal reference signal spanning the same subcarriers can be generated by adding a linear phase rotation to the same base reference signal. By using different phase rotations for different user terminals, a large number of mutually orthogonal reference signals spanning the same subcarriers can be generated. Currently, there is no method of ensuring mutual orthogonality of the reference signals in the scenario where the user terminals are allocated different but overlapping subcarriers.